High-frequency inductance



Oct. 30, I934.

Res. l N OHMSPER. MlCROHE-NRY.

Filed April 9, 1934 00 900 moo 1500 12:00 2000 FREQ. IN KILOCYCLE'S ALFRED C'eossun;

(HA ELES 6. IVE/6H5 0E5 ATTORNEY.

INVENTORS,

Petented 30, I v v -g 8,5 8

u lTEo STATES; PATENT oFFlcs 1,978,568 HIGH-FREQUENCY INDUCTANCE inurea Crossley and Charles 0. Neighbors, Chicago, 111., assignors 'to Johnson Laboratories,

Inc., Chicago, 111., a corporation of Illi v f u Av 9, 34. Serial No. '119,'l37

9 Claims. (01. 175-356) I The present invention relates to inductances is capable of application under a wide variety of intended for use in high-frequency circuits and conditions as to each of the principal electrical more particularly to that class of high-frequency dimensions of the inductance itself, and as to the inductances in which it is essential to reduce the range of frequencies over which it may be ad-.

dissipative energy losses to the lowest possible vantageously employed. For any particular set 60 value. Inductances of this class, for example, of conditions, it will be found that our invenare essential in high-frequency measuring aption provides a method of determining all the parat'us, and are used extensively in wireless, radio physical dimensions and thus the complete design and carrier signaling systems. for an inductance having the best operating char- The invention herein-disclosed has particular acteristics that it has thus far been possible to 65v applicability in those cases in which the current attain. In each case, the inductance designed flowing through the inductance is not of such in accordance with the teaching of the present magnitude as to determine the size of the conspecification will have more desirable operating ductor necessary to avoid excessive heating of the characteristics at any particular frequency, and,

inductan e device, I additionally, a new behavior of its characteristics, 70 It is a principal object of the present inventionas the freque Varied OVBIT a nsiderable t provide a form of higmfrequency mduetance range, differing essentially .from that attained in having a considerably greater ratio ,of d sir d any earlier form of inductance device. inductance to undesired dissipative losses than In ycasesit iS desirable to o e high- '20 any form of high-frequency inductance at present' frequ y e-s or the devices in which 75 or heretofore employed. Further objects of the they a ed, in metallic shields, in order t0 P s present invention are to provide a form "of innt other compcnents'of the app tus rom ductance inexpensive to construct, whose. comaffecting them, and t0 nfi ethe magnetic and ponents are capable of manufacture on automatic electric fie ds of e ducmncefitself. One of machines, and which will have negligible or readth u qu a a st s of u pr v d hi h- 80 ily compensated variations in its parameters when f q y inductance is hat it m y be enclosed produced in quantities. Y in smaller shields than would be necessary to As will be apparent from what is to follow, avoid exccssivelcsses e inductencecf the these and other'desirable objects are attained Ventiohal typ e though t e ma um diamin the present invention by employing an entirely t r of ur i ro indu i y e eat r 85 new component. in the construction of uch inthan the diameter of'the conventional inductance ductances. This new. component completely modihaving the Same inductance Vfliueties the conditions which determine the most de- Th v nt n p vid s am f d terminin sirable dimensions of the coil itself, enables forms id dimensions f r inductance or y of winding to be used which heretofore have been particular Set design wilditions- 111 general, 90 considered undesirable, and results in an inoweve it will be found desira t p Slight: ductance entirely novel in its performance ca- 15 from these ideal dimensions inorderto accompabiities and in its adaptability to apparatus in modate h design to h Par i ul r conditions of which such components are employed. manufacture under which it i to be P c d- The range of frequencies commercially utilized F p i may a o mploy 95 'in wireless and radio, and in carrier-frequency Sizes 0 yp of fl d fl differing rom those communication, and for which the new high-fre which would produce the best u t to emquency inductance is adaptable, may be taken ploy dimensions'for the windings or for other as extehding from approximately 30,000 cycles parts of the design which are accommodated to per second .to 3,000,000 cycles per second. For the particular tool andmachine equipment avail- ,I

the purposes o1.this specification, the range ot-able for their manufacture. It be underfrequencies from 30,000 to 100,000 cycles per secstood, therefore, that these departures are within and may betaken as the low high-frequency the purview ot the present invention. v range, the range from 100,000 to 500,000 may be ,For a complete understandingoi the invention,

taken as the medium high-frequency range and .the following description should be read in conm5 the range from 500,000 to 3,000,000 may be taken nection with the drawing, in whichas the upper high-frequency range, each of these Fig. 1 is a view in perspective of a generalized h ranges being taken as having reasonable'overlap high-frequency inductance exempliiyin'g the ineach into the next. .vention; Y

our new improved high-frequency inductance Fig.2 isaditic drawing or such ahighindicated as a and b, respectively;

Fig. 3 is a view in perspective showing a highfrequency inductance embodying the invention and designed particularly for use in the upper high-frequency range; I Y

Fig. 4 is a view in perspective of another highfrequency inductance embodying the invention and designed particularly for use in the medium high-frequency range; and

Fig. 5 is a graph showing the relation between the high-frequency resistance of inductances constructed in accordance with the invention, and frequency.

--Referring to Fig. 1, it will be seen that our improved high-frequency inductance consists essentially of two parts, namely, a coil 1 and a core 2. The coil 1 is preferably of the type sometimes designated as a lattice winding or universal winding. Coils of this description are well known in the art and are conveniently wound on ma.-

chinery especially designed for the purpose. They are particularly characterized by" the fact that turns in any one layer do not lie parallel to turns in the layers next above or below. The advantages of this type of winding, as well as additional precautions to be employed in utilizing it, will be more clearly understood from what follows.

Still referring to Fig. 1, the core 2 is an iron core containing individually-insulated and extremely minute magnetic particles, compressed with a suitable binder to a desired apparent permeability. Magnetic material of this character and processes for producing it have recently been described in British patents, Numbers 366,475 and 403,368, and in other patents and publications,.both in the United States and Europe.

The invention resides, not in the forms of winding which are used or in the magnetic material employed in the construction of the magnetic cores, but rather. in the manner in which the winding and the core cooperate, because of their dimensions and proportions and because of their physical relationship, one to another.

Iron-core inductors were employed in the early days of the art of wireless telegraphy. In general, however, their use was restricted to the low high-frequency range and -the iron cores were constructed of laminations. In order to reduce the energy losses in these iron cores, the thickness of the laminations was reduced, in some cases, to as little as 0.001 inch. Since sheets of iron-containing metal thinner than this could not be produced at reasonable expense, and in order to still further reduce the losses, cores of powdered ferrous material were employed. These ferrous powders were either held in a suitable container to form a core body, or they were held together in a solid mass of suitable shape by employing various binding materials. In no case, however, so far as applicants are informed, until the disclosures of the British patents above referred to, were the ferrous particles employed in these cores individually insulated and of the extreme degree of fineness and the high degree of purity of the iron content, which are contemplated in the present in-.

vention." 'Th'e'p'articles in these earlier cores were not individually insulated, or if they were individually insulated, the material used for the insulation was of such a nature as to preclude the attainment of the extremely low'order of losses herein contemplated.

Following these early experiments with ironcore inductances, and because they were unsuc- 7 1,978,668 frequency inductance, showing two views thereof,

cessful and further because, even as then constructed, air-core coils were greatly superior to the best that could be secured .in iron-core inductances, at least in the medium and upper high-frequency ranges, those who were working in ,the art of. high-frequency signaling more or less completely abandoned any attempt to produce eifective iron-core high-frequency induc-' ances, and concentrated their attention upon the development of increasingly eflicient air-core inductances.

It was well established as early as 1918, that the most efllcient form of air-core inductance for high-frequency purposes was asingle-layer solenoid having a ratio .of diameter to length of approximately 2.46 and that the best form of conductor for use in winding such a solenoid was one having a number of insulated strands. Further development of these 'inductances demonbeen away from the ideal proportions, so that modern radio receivers employ single-layer solenoids having a ratio of diameter to length as low as 0.4.

The intermediate-frequency. amplifiers of superheterodyne receivers are normally-designed to operate at a frequency lying in the range which has been designated as the medium high-frequency range. Because these amplifiers are designed to operate at a fixed frequency, and also because of the very high amplifying capabilities '.of modern thermionic relays, it is possible to utilize air-core inductances of types having relatively low efficiency, which could not be successfully utilized in the resonant circuits of high-' frequency amplifiers intended to be tuned over a band of frequencies in the range which has been designated as the upper high-frequency range. Thus, types of windings, without iron cores, similar in their general aspects to the windings contemplated in the present invention and designated as lattice windings or universal windings have been employed in these intermediatefrequency amplifiers. This use of coils of this type is not to be confused with theuse in our improved high-frequency inductance, of coils of similar character and similarly produced. This is because, as will be apparent, the employment of a ferro-magnetic core completely alters-the problems of the design and'the physical dimensions ofthe resulting high-frequency inductance.

Remembering that an inductance is designed to have a particular desired inductance value stated in microhenries or millihenries, it is to be noted that twoother electrical dimensions of the device must be contemplated in the design and play a controllingpart in determining its utility.

the surface of a conductor;

cases, iron and other wires having higher resistance than copper have been employed; Because 'of the minute electric currents produced in any one turn of a coil by the magnetic field generated in the same turn and in adjacent turns, highfrequency currents can travel, in general, only on The eddy currents in the wire caused by these magnetic fields, generate counter electro-motive forces which prevent the. current from flowing in the center'por- 'tions of the wire. This materially increases the effective resistance of the conductor, the efiect i n-.-

creasing as the frequency increases.

A second consideration is thetype of insulation used, not only for surrounding the conductor itself, .but also to form the support upon .which the coil is wound. Because of the electrostatic field which exists between turns in a coil due to the differences in voltage, there are, in general, dielectric losses in the insulating material. These losses may be minimized by reducing the amount of insulating material necessary to insulate the turns of the coil, andto support it,-and by employing insulating materials having lowinherent amount of dielectric material may also have been reduced, because it increases the intensity of the electrostatic field existing between turns of the coil.

In our iron-core inductance, however, due to the effective permeability of the core; the length of wire necessary to produce a desired value of inductance is materially decreased. This oper ates to decrease the losses in the wire itself, without any increase in the losses in the insulating material because, in general, a smaller amount of it is employed. Against this, however, the iron itself introducesnew losses and, therefore, tends to increase the effective resistance of the complete device. The successful employment of an iron core, therefore, requires that it shall be of such a nature that the losses which itintroduces are materially less than the losses eliminated by virtue of the smaller amount of wire and insu-- lating material required for the given inductance value.

The high-frequency resistance of the wire, .the losses in the insulating material, and the losses in the iron core, all increase with frequency. The

' losses in the irony core increasesomewhat more rapidly with frequency than the losses in the wire and the insulating material. In .the lower higha frequency range, the increase inlosses due to the presence of the core is very much less than the decrease in .losses resulting from the use of a 7 material for the same inductance value.

materially smaller amount of wire and insulating I In this range, therefore, theiron-core inductance of the .present invention has a very much higher em- 'ciency than an air-core inductance having the same inductance value. As' the frequency-is increased, with corresponding increase in losses, the

advantage secured by the use of the iron core winding machines.

gradually diminishes, but throughout the lower, medium, and upper high-frequency ranges, the iron-core high-frequency inductance of the present invention is superior to an air-core inductance havingthe same inductance value.

As anexample of the advantage just described,

it may be pointed out that a high-frequency inductance embodying the present invention, when measured at a frequency of 2,000,000 cycles per second, will have '70 greater inductance, but only 26% greater high-frequency resistance than the same winding without the iron core. The reactance to resistance ratio of such an inductance, that is, the "Q value commonly used by engineers in the high-frequency art, will be of the order of 165 at a frequency of 1,200,000 cycles per second, as against a value of 144 for an air-core inductance constructed in accordance with the most advanced principles of conventional design,

at the same frequency. At 600,000 cycles per second, the Qv of the same iron-core coil has risen to 214, while the Q of the air-core coil has'fallen to 140. At this frequency the iron core coil is 53% better. q

The distributed capacity of a winding is injurious in two ways. In the first place, the higher the capacity is, the greater will be the loss, re-

quency may be very troublesome inparticular applications.

Since by employing an iron core having suitable loss characteristics, the length of wire in the coil necessary to produce a particular inductance value may be materially decreased, itis possible at the same time to. reduce the distributed capacity of the coil, by decreasing the length of the coil. The reduction of the distributed capacity, materially increases the resonantfrequency of the coil and also decreases the dielectric losses in the insulating material, which; appear as a portion of the effective resistance of the coil.

As has. been pointed out, the magnitude of the current which traverses high-frequency inductance devices, herein contemplated, is so small that it does not determine the size of the wire necessary. The coils are therefore wound with 1 0 a wirewhich is mechanically convenient and which can be successfully used on universal For inductances for use in the low high-frequency range, solid wire may be-used, such for example as No. 38 silk enamel.

In the medium and upper high-frequency ranges, wire having a plurality of enamel insulated strands, with single silk insulation overall, is to be preferred. Since the size of the wire is not determined by the current, the strands may be of No. 41 SB. 8: S. gauge, which has been found to be mechanically feasible, and satisfactory in use: In the medium high-frequency range, seven such strands may be used,.but in the upper high-frequency range it is preferable to 14,5

use ten such strands. It will be understood that the preferred proportions of the coil are to a slight extent modified if wire ofother sizes is It is preferable to usea wire having silk insulation over the bundle of strands,since this 1&0

is found satisfactory .for use on the winding machines and gives adequate insulation between the turns with lowv dielectric loss. The use of some other insulation would also affect, in some slight degree, the ideal dimensions of the coil.

For the lower and medium high-frequency ranges, the wire may be close wound, but in in ductances to be used at frequencies in the upper high-frequency range, the winding is preferably arranged to leave a space between the turns substantially equal to the overall diameter of the stranded conductor, for the purpose of reducing the capacity between contiguous portions of a. turn.' Any greater spacing than this would slightly aifectv the ideal dimensions and would not materially assist in decreasing the capacity. So far as the cross-over of the winding is concerned, the ideal dimensions for inductances to be used in the medium high-frequency range contemplate that the angular distance in which each convolution of the winding passe. from one side of the coil to the other and back again, will be slightly over 360 degrees. In the upper highfrequency range the number of cross-overs in 360 degrees may advantageously be increased to as many'as six. Any variation in the cross-overs will also have 'aslight effect upon the ideal pro.-

portions of the winding.

It is preferable to wind the coils directly upon the core, using paper or other suitable insulating material having a thickness of approximatew frequency inductances are, in general, inversely proportional to the frequency at which they are to be employed. For example, at an intermediate frequency of 175,000 cycles it is customary "to employ an inductance of approximately 8,000

microhenries-with a tuning capacity of about 120 micromicrofarads. At an intermediate frequency of 456,000 cycles, however, the usual inductance value will be of the order of 1,500 microhenries, with a tuning capacityof about '70 micromicrofarads. For use in the broadcast range of frequencies, that is, from 550,000 cycles to 1,500,000 cycles, the usual value of inductance is about 250' microhenries, and it is tuned to, the highest frequency in the range with a minimum capacity of approximately 30 micromicrofarads.

It will be understood, therefore, that in stating the proportions which determine the design of our improved high-frequency inductance for any desired inductance value, we have in mind that the. inductance values will, in any particular case, be of the order of the values 110W commomy 4 employed. If, however, it is desired to design an inductance in accordance with this invention, but having an inductance value widely different from the value commonly employed at the intended frequency or frequency-range, it will be found that the proportions to be given will still determine the best design, but that, in gen-- eral, it will be preferable to choose a size of wire which will give the required inductance value within the design proportions. For example, if amuch larger inductance value than is cometer of the conductor, or the number of strands,

or the size of the individual strands, or the space betweenthe turns, may be increased, to again maintain the proportions of the design.

Because the iron core partakes of the characteristics of. both an insulator and a conductor, it isessential to so arrange the design that the turns of the winding are contiguous to the core to the minimum feasible degree. For this purpose, a lattice oruniversal winding having a ratio of length to radial depth considerably less than unity is advantageous. Referring to Fig. 2, depending on the inductancevalue' and the fre-. quency at which the inductance is to be used, the ideal shape of thewinding is given by the expression: .375 C .66D.

Referring again to Fig. 2, it will be observed that the iron core is centrally located in the hole in-the coil and extends slightly to either side. The ideal length for the core may be expressed as follows: .75A B 1.5A. If the length is ma-. terially decreased below the lower limit given above, the apparent permeability of the core will diminish and a greater number of turns,'representing more wire and greaterlosses, will be required to produce a given inductance value. If

the length of the core is-increased materially beyond the maximum given above, there will be only a small possible reduction in the amount of wire for a given-inductance value, while the losses due to the core will be disproportionately increased.

, The ideal relation between the depth of the winding'and the external diameter of the core maybe expressed as follows: .4A D .9A. Between these limits the complete inductance will have minimum capacity and minimum resistance for a given inductance value.- In cases where it is desired to have the inductance resonant at some particular frequency, a higher distributed capacity can be secured by increasing the diam-f eter of the core or by decreasing the number of turns on the coil. On the other hand, if extremely low distributed capacityis desired, at the expense of a slight increase in resistance, this result may be secured by decreasing the length -of the coil and increasing the number ofturns on the coil.

a The core 2 in Fig.2 is shown as being tubular. Theory indicates and'experiment proves that the central, portion of a solid core has only a very slight efiect in increasing the apparent permeability. By making the core tubular, a smaller amount of magnetic material is required and the completed inductance is provided with a convenient hole for ready mounting upon any suit-.

losses. Decreasing the. wall thickness of the core beyond the proportions above given, however,

materially decreases the apparentpermeability of above stated, and embodying our invention, the following data is given:

' Frequency 4'56 kilocycles Inductance '.l. 1,500 microhenries Resistance; 27 ohms Capacity 2.9 micromicrofarads Reactance to resistance ratio, Q=wL/R 160 External diameter of core .375 inch 1 Length of core .500 inch Wall thickness of core .087 inch Length of winding .120 inch Depth of winding .292 inch Number of turns 240 -Conductor:

Number of strands 'I Sizeof strands No. 41 enameled Insulation over strands Single silk Winding:

Space between turns l None Throw (times across and back per turn) 1' in the losses.

As anexample of ahigh-frequency inductance designed .in accordance with the proportions As'will be apparent from the discussion above given, the presence of the iron 'core materially around the coil.

alters the distribution of the magnetic field If the core were not present, or if it were only of the same length as the coil, the flux lines would lie much closer to the sides of the coil, thus increasing the eddy-current losses in the turns of the coil itself, and decreasing the efiective inductance. The presence of a core of the proportions shown decreases the con-' centration of flux adjacent the turns of the coil and thereby tends to keep the losses due to the eddy'currents in the turns of the coil at a rhinimum.

In order tomake clear the advantage of designing the core in accordance with the proportions given, it may be pointed-put that the same amount of magnetic material formed into a spool extending up both sides of thelooil and through ed with the ideal proportions. Additionally, the

presence of, the iron close to aconsiderable number of the turns on the coil would materially increasethe-dielectric losses and thus still further increase the high-frequency resistance of the device.

It will be understood, as already explained, that departure maybe made from any of the proportions herein stated and that inductances may be designed, utilizing the above relations, but departing from the proportion or the wire sizes and types above indicated, without departing from the scope of the present invention. Fig. 3 shows an inductance designed in ac-' eordance with the aboveprinciples and intended particularly for use in the-upper high-frequency range, for example, the broadcast. frequency range, the drawing being closely to scale. It will be noted that in'thispase a somewhat larger diameter of iron core has been used and that the radial depth of the winding is correspondingly decreased. It will be understood that a coilsuch as that shown in Fig. 3 represents an embodiment of the invention in which a deliberate departure fromwhat may be regarded as the ideal proportions has been made, in order; to favor the specific characteristics desired in a particular inductance.

Fig. 4, similarly to Fig. 3, shows another embodiment of the invention, intended particularly for'use in the medium high-frequency. range, such for example as the frequencies commonly employed in the intermediate-frequency amplifiers of superheterodyne receivers. Here again deliberate departure from the proportions which may be regarded as ideal has been made, in order to favor specific characteristics of the inductance for this specific case.

Curve A, in Fig. 5 shows the resistance per unit of inductance (ohms per microhenry), over the frequency range from 300,000 to 2,000,000 cycles per second for an inductance designed in accordance with this invention. For comparison, curve B shows the resistance per unit of inductance for the same inductance without the iron core.

The inductance without the core, in this case, was

148 microhenries, whereas with the core it was 254 microhenries, an increase of 70 As will be seen from these curves, the resistance, in ohms per miorohenry; even at the highest frequency, namely, 2,000,000 cycles per second, was 26% lower for the iron-core inductance.

Windings of the type described inthis specification are multiayer windings, and in accordance with the proportions given will have at least twice as many layers as turns'p'er layer. The

'cores are preferably of circular cross section as shown, but may be of polygonal cross section, and will produce inductances departing from .the proportions given to an extent depending upon how greatly the section of the core departs from a circle. The coil may be mounted otherwise than centrally of the core, the resulting inductance departing from the proportions given to an extent depending upori the eccentricity. These and other variations from, the proportions indicated may be made without departing from the scope of the present invention.-

In preferred embodiments of the invention, the coil is secured centra ly of the core with any suitable adhesive, and the assembled high-frequency inductance is then given a flash dip of any suitable water resistant wax to exclude moisture, and to protect the winding from mechanical damage. The inductance may be mounted upon any suitable insulating post or pin, but metallic mountings should be avoided, since they inevitably increase. the'losses, and the apparent resist-- ance of the inductance. and may also operate to increaseits capacity. The ends of the winding may conveniently be used as leads for making electrical connections to the inductance, but suitable terminals are preferably provided near the inductance, so that the relatively smallwire will .not'extend unsupported for more than a shortdistance. I

Having thus described our invention, what we -claim'is: v 1." A high frequency inductance device having a ferrosmagnetic core and a winding, said winding having a length not greater than six tenths the diameter of said core and being wound centrally thereof with a multi-layer winding having a depth at least one and one-half times its length.

2-. A high-frequency inductance deviceincluding a concentrated winding having a'number of layers at least equal to the number of turns per layer and an iron-containing core having a length not less than two of its diameter but length, and a tubular ferro-magnetic core, said winding being centrally positioned upon and concentric with said core, the internal diameter, external diameter and length of said core, and the internal and external diameters of said winding being substantially proportional to the numbers two, five, flve, five and ten respectively.

4. A high-frequency inductance comprising a multi-layer self-supporting winding having a radial depth substantially two and four-tenths times its length, and a magnetically cooperating core, said winding being centrally secured upon and concentric with said core, said core having a length approximately one and one-third times its diameter and flve tenths the external diameter of said winding.

5. A high-frequency inductance device including a pancake winding having at least as many layers as turns per layer, and a tubular cooperating iron-containing core, said winding being mounted upon and concentric with said core, the thickness of the wall of said core, the external diameter and length of said core and the length six respectively."

and radial depth of said winding varying by not more than 25% from proportionality to the numbers twelve, forty, forty-six, fifteen and twenty- Patent No. -l, 978, 568.

. 6. A high-frequency inductance including'a multi-layer winding and a term-magnetic core,

with respect to each other that the length of said winding is not less than fifteen one-hundredths of the diameter and two-tenths of the length of said-core.

'7. A high-frequency inductance device including a multi-layer winding and a term-magnetic core, said winding and said core being so proportioned with respect to each other that the length of said winding is not less than fifteen one-hundredths of the diameter and two tenths of the length of said core and three eighths or the radial depth of said winding.

- 8. A high-frequency inductance device including a multi-layer winding and a term-magnetic core, said winding and said core being so propor tioned with respect to each other that the length of said winding is not more than six tenths of the diameter and .four tenths of the lengthof saidcore.

9. A high-frequency inductance ing a multi-layer winding and a ferro-magnetic core, said winding and said core being so proportioned with respect to each other that the length of said winding is. not more than four tenths of the length and six tenths of the diameter of said core and not less than three eighths of the radial depth of said'winding.

ALFRED cnossnnr. CHARLES c. NEIGHBORS.

. CERTIFICAlE 0F CORRECTION.

October 30,- "1934.

ALFRED enossmr; ET AL.

It is hereby certified that error appears in the printed specifica tioii of the abovenumbered patent requiring correction as follows: Page 4, line 98, for ".375" read .3750; and that the said Letters Patent should be read with this corr'ection therein that the samemay conform to the recordoi the case Signed and sealed this 18th day of June, A.-D. l935.

in 'the Patent Office.

. Leslie Frazer Acting Connnissioner of Patents.

DISCLAIM ER 1,97 8,568.Alfred Crossle'y-and Charles C. Neighbors, Chicago; Ill. HIGH-FREQUENCY INDUCTANCE.

Patent dated October 30, 1934 disclaimer filed August 27,

1938, by the assignee, Johnson Laboratories, I Hereb enters this disclaimer to claim 5 in said specification.

z'al Gazette September-20, 1.938.]

device includ-' said winding and said core being so proportioned 

